Distillation is an important method for the separation of fluid mixtures in industries such as petroleum refining, organic and inorganic chemicals production, and the separation of atmospheric gases. Distillation is an energy intensive process, especially in the separation of low-boiling gas mixtures associated with cryogenic air separation, nitrogen rejection from natural gas, synthesis gas separation, and the separation of light hydrocarbons. In these separations, mechanical or electrical energy is utilized to supply the large amounts of driving force required to operate the separation equipment at temperatures far below ambient. It is desirable to improve the energy efficiency in such separations in order to improve the economics of recovering low-boiling gas products.
Improved separation of such low-boiling mixtures has been achieved in the art by the use of multiple heat-integrated distillation columns. For example, in many cryogenic air separation processes two columns are operated at different pressures and are thermally linked such that condensing vapor at the top of the higher pressure column provides heat through indirect heat exchange for vapor boilup at the bottom of the lower pressure column. Such a method requires that the temperature difference between the heat source provided at the bottoms reboiler of the higher pressure column and the refrigeration provided to remove heat at the top condenser of the lower pressure column be much greater than the same temperature difference in a single column distillation process. Double-column distillation systems are well-known and widely used in the cryogenic separation of air.
The use of a high pressure column with a low pressure column to reduce heat duty in distillation has been studied extensively in the art. Representative descriptions of such systems are given by A. W. Westerberg in a review article entitled "The Synthesis of Distillation-Based Separation Systems" in Computers and Chemical Engineering Vol. 9, No. 5, pp. 421-429, 1985 and in an article by N. A. Carlberg and A. W. Westerberg entitled "Temperature-Heat Diagrams for Complex Columns. 2. Underwood's Method for Side Strippers and Enrichers" in Ind. Eng. Chem. Res. 1989, 28, pp. 1379-1386. A characteristic of the systems described in these articles is that heat duty in the distillation columns is reduced by providing heat to the higher pressure column at a higher temperature and heat to the lower pressure column at a lower temperature. This characteristic results in the operation of the higher pressure column at temperatures greater than the temperatures in the lower pressure column.
Cryogenic air separation systems can be integrated readily with combustion turbines, particularly in combination with the generation of electric power in combined cycle processes. The combustion turbine air compressor can provide compressed air for the turbine combustor as well as for the air separation system, and pressurized waste gas (typically nitrogen-rich) from the air separation system can be introduced into the combustor or expansion turbine to recover pressure energy and increase the overall system efficiency. Double-column air separation systems have been integrated with combustion turbines as disclosed in representative U.S. Pat. Nos. 4,224,045, 5,081,845, 5,251,451, and 5,257,504.
Further improvement to the efficiency of cryogenic air separation systems can be realized by utilizing three integrated columns operating at three different pressures. Such systems are particularly useful when oxygen and/or nitrogen products are required at elevated pressures. U.S. Pat. No. 5,231,837 discloses a triple column air separation system in which an intermediate pressure column operates between the high and low pressure columns. The intermediate pressure column is a stripping column fed at the top with partially vaporized liquid from the bottom of the high pressure column; reboiler duty to the intermediate pressure column is supplied by indirect heat exchange with vapor from the top of the high pressure column. Vaporized bottoms and flashed overhead condensate from the intermediate pressure column are fed to the low pressure column. A low pressure oxygen product, a high pressure nitrogen product, and a low pressure nitrogen product are recovered from the process. The process optionally is integrated with a combustion turbine wherein the two nitrogen product streams are compressed and introduced into the turbine combustor.
U.S. Pat. No. 5,341,646 discloses a triple column air separation system in which feed to the intermediate pressure column is provided by both the overhead and bottom streams from the high pressure column and by a stream of cooled and partially condensed air. The intermediate column contains both rectification and stripping sections, and reboiler duty to the column is provided by compressed overhead vapor from the high pressure column.
The potential for efficiency improvement in the separation of low-boiling gases is particularly favorable when the feed composition is such that the mole fraction of the desired lighter (more volatile) component to be recovered is significantly different from that of the desired heavier (less volatile) component to be recovered from the feed mixture, and when the products are required at elevated pressures. This combination of conditions is particularly applicable, for example, to air separation systems which supply oxygen at elevated pressures to hydrocarbon gasification processes operated in conjunction with combined cycle power generation systems. Multiple-column air separation systems with increased operating efficiency are desirable for use with such combined cycle power generation systems, and an improved triple-column air separation system for such an application is described in the following specification and appended claims.